This invention relates generally to epitaxial fabrication of semiconductor structures and, more particularly, to processes for patterning metal connections on semiconductor structures. In bipolar semiconductor circuitry, aluminum or a similar material is typically used as a metallic conductive layer, to establish connections with underlaying semiconductor layers. Contact regions, with which electrical connection are to be established, are capped with a contact material, such as platinum silicide, before the aluminum layer is applied. Non-contact regions are coated with a protective layer of silicon dioxide before the aluminum layer is applied; then the aluminum is selectively removed from the non-contact regions.
A well known difficulty in the processing of aluminum metallization layers .in this way is that the aluminum, in contact with such materials as platinum silicide will penetrate the contact material when the structure is heated. To prevent this unwanted penetration of the aluminum, a common practice is to employ an alloy of titanium and tungsten (TiW) beneath the aluminum layer. The TiW material does not penetrate the contact regions, and can also serve as an etch stop layer during removal of the aluminum from the non-contact regions. For example, U.S. Pat. No. 4,267,012 to Pierce et al. describes a process in which a TiW alloy is used as an etch stop layer for the removal of undesired portions of an aluminum layer. The TiW layer is then itself removed by an etching process, such as wet chemical etching.
In recent years, the lateral dimensions and spacings of semiconductor regions have become smaller and smaller, as fabrication techniques have improved and device densities have increased. With the decrease in lateral dimensions, usually referred to as device "geometry," layer thicknesses have also become smaller. In particular, the protective layers of silicon dioxide formed over non-contact regions may be so thin that the removal of the TiW stop etch layer becomes a much more critical process step. A strong wet etch may eat significantly into the silicon dioxide. Plasma etching materials available for etching the TiW alloy are, unfortunately, not selective with respect to silicon dioxide. Once the TiW layer is etched away, the plasma etch will immediately begin attacking the silicon dioxide, and underlying active devices in the structure can be exposed. Timing of the etch step is, therefore, very critical and practically impossible to control if the silicon dioxide layer is extremely thin. This processing difficulty effectively limits the thickness of the silicon dioxide layer and, indirectly, the attainable device geometry.
It will be appreciated from the foregoing that there is a need for improvement in the process steps of applying a metallization layer to semiconductor structures. The present invention is directed to this end.